There has been a growth in demand for packet switched wireless data services due to the growth in internet applications. A typical channel over which these data services are delivered is a radio channel. Radio channels are available in an increasing number of frequency bands. A frequency band of particular interest is the IMT-2000 frequency band (at a frequency of about 2 GHz). This frequency band is used for delivery of data services using wideband code division multiple access (WCDMA) techniques. Two WCDMA techniques that may be used in this frequency band are frequency division duplex (FDD) techniques and time division duplex (TDD) techniques.
A strategy that is useful when packet data services are transmitted across a fading radio channel is to exploit multi-user diversity. Multi-user diversity may be employed when there are multiple users that are all requesting service concurrently. If the transmitter knows the channel conditions that are being experienced by the receivers that it is serving, it may schedule those users that are experiencing favorable channel conditions in preference to those experiencing unfavorable channel conditions. Furthermore, the scheduler may wish to use less error correction coding or transmit using a higher order modulation when transmitting to users with the better channel conditions (such techniques will increase the instantaneous throughput to those users).
In the 3rd Generation Partnership Project (3GPP) communication systems utilizing packet data services and employing High Speed Downlink Packet Access (HSDPA), the transmitter is referred to as a Node-B (i.e. a ‘base station’) and the receiver is the user equipment (UE) (often referred to as a ‘remote station’ or ‘subscriber equipment’). The HSDPA system that is specified by 3GPP exploits multi-user diversity in several ways:                The amount of error correcting, coding and modulation applied may be varied between transmissions (for example when applying adaptive modulation and coding (AMC)).        A scheduling function is located in the Node-B. This network element has a shorter round trip delay to the UE than the RNC (Radio Network Controller), which is where the scheduling function is classically located. The Node-B may attempt to always choose users to schedule that are experiencing favorable channel conditions.        The UE reports channel quality directly to the Node-B, allowing the Node-B to make scheduling decisions based on channel quality.        
3GPP have specified HSDPA both for the FDD (Frequency Division Duplex) and for the TDD (Time Division Duplex) modes of operation. In both modes of operation, there is a mechanism by which channel quality estimates are fed back from the UE to the Node-B.
In current specifications of HSDPA, notably with respect to a format of medium access control (MAC) layer protocol data units (PDUs), only MAC-hs (high speed) Service Data Units (SDUs) from a single priority queue of a UE can be multiplexed onto one MAC-hs PDU. As illustrated in FIG. 1, a single MAC-hs PDU 120 may be sent to a user equipment (UE), per transmission time interval (TTI). The single MAC-hs PDU contains a MAC-hs header 125, followed by MAC-hs payload 130 (comprising one or more MAC-d PDUs, where a MAC-d pdu is the same as a MAC-hs SDU, but where MAC-d layer is located above the MAC-hs layer in the 3GPP architecture) and finally optional padding 135, if the sum of the above data does not fit a valid MAC-hs PDU size (where the permitted size values are defined in 3GPP TS 25.321, known as ‘k’ values).
Further, in 100 a size of a single MAC-hs PDU is signalled 115 in the high-speed shared control channel (HS-SCCH) 105. As a consequence of only being able to transmit a single MAC-hs PDU 120 to a UE, only a single priority level of data can be carried.
However, in the MAC-hs logic entity at the network (UMTS Radio Access Network (UTRAN)) side of the communication link, more than one priority queue may belong to the same UE. In effect, this means that if one UE is scheduled to transmit in a TTI, only MAC-hs SDUs from one of the priority queues of the UE can be sent in one MAC-hs PDU. This is the case even though downlink resources in this TTI may allow for transmitting more MAC-hs SDUs than those that are waiting for transmission in any one of the priority queues of this UE.
It has been proposed by Huawei in a document titled ‘Concatenated MAC-hs PDU’ in 3GPP TSG-RAN-WG2 meeting#51 in Denver, USA, Nov. 13-17, 2006, to concatenate MAC-hs PDUs in the manner illustrated in FIG. 2. As shown in FIG. 2, a concatenated MAC-hs PDU 250 includes multiple MAC-hs PDUs 205, 225, 230 built from different priority queues of the same UE. The format of each MAC-hs PDU follows current specifications, except that there is no padding field since the MAC-hs PDU size is not constrained by the ‘k’ values; it comprises a MAC-hs header 210 followed by multiple MAC-hs SDUs 215, 220. Since possible padding may be needed for the concatenated PDU, an optional pointer field 240 with typical length of ‘8’ bits or ‘12’ bits is used to indicate the beginning of the padding field 235. A fixed PF (Pointer Flag) 245 with one bit length is located at the end of the concatenated MAC-hs PDU 250, and is used as an indicator to show if the pointer field 240 is present (for example if PF=‘1’) or not (for example PF=‘0’).
There are a number of drawbacks associated with the concatenated PDU proposal outlined above, including:                Existing MAC-hs software developed according to 3GPP cannot be re-used:                    The individual MAC-hs PDU structure differs from the existing 3GPP structure            A Concatenated MAC-hs PDU has a new structure that is also handled by MAC-hs.                        
Therefore, a desire exists for HSDPA communication wherein one or more of the abovementioned disadvantage(s) may be alleviated.